1. Field of the Invention
The present invention relates to materials for optical communications, and more particularly, to a polyesterimide for optical communications, which minimizes optical loss in a near infrared wavelength range.
2. Description of the Related Art
A wavelength range for optical communications has been shifted from 800 nm to 1550 nm, which corresponds to the near infrared wavelength range. Thus, it is ideal to manufacture an optical communication device using a material which barely absorbs light belonging to the wavelengths of the near infrared range.
A polymer is generally used for an optical substrate such as an optical lens or compact disk. Recently, many attempts have been made to use such polymers as optical waveguide materials for light transfer in the near infrared wavelength range.
A conventional polymer generally absorbs light of 1000-1700 nm which corresponds to the near infrared wavelength range. Such absorption of light in the near infrared wvavelength range by the polymer is caused by overtone of harmonics due to stretching and deformation vibrations of carbon-hydrogen (C--H) bonds in alkyl phenyl and other similar finctional groups. Thus, it is not desirable to use such a conventional polymer as the optical waveguide material utilizng the light of the near infrared wavelength range because of a large optical loss. In order to reduce the optical loss, the light absorption wavelength region of a polymer must be shifted from the near infrared wavelength range to a longer or shorter wavelength region. To this end, a method in which hydrogen in the C--H bond is substituted by fluorine (F) or deuterium (D) has been suggested.
Particularly, in the case of substituting hydrogen with deuterium, a C--D bond causes light absorption at the wavelength range of about 1500 nm. Deuterium-substituted polymers are therefore not suitable for materials for optical communications devices using the 1500 nm wavelength range. On the other hand, substitution of hydrogen by fluorine can minimize optical loss in light absorption at a wavelength in the range of 1000-1700 nm.
An optical material used for fabricating optical devices such as an opto-electronic integrated circuit (OEIC), an opto-electrical mixed wiring board (OEMWB), a hybrid integration device, a plastic optical fiber or a multi-chip module (CM) must have good thermal stability during a fabrication process, particularly at a temperature of about 250.degree. C. Since the thermal stability of an optical material is a very important factor, careful consideration must be taken of the glass transition temperature, thermal decomposition temperature, thermal expansion coefficient or birefingence of the optical material.
Polyimide has been most widely known as a polymer having good thermal stability. Since polyimide is stable at a high temperature of about 400.degree. C., great efforts to utilize polyimide as a material for optical communications have been consistently made.
However, generally, since a conventional polyimide has many C--H bonds in its molecule structure, it exhibits a large optical loss in the near infrared region To overcome this problem, recently, a method in which hydrogen in C--H bonds of a polyimide is partially or entirely substituted by fluorine has been proposed.
However, if hydrogen is substituted by fluorine, the refractive index of the polymer is decreased. Here, the content of fluorine in the polymer is proportional to the decreased level of the refractive index. Thus, since a polyimide obtained by substituting hydrogen in the C--H bonds by is fluorine, that is, a fluorinated polyimide, has a low refractive index, in the case of using the fluorinated polymer as a core, the range of selection of materials capable of being used for cladding becomes narrow.
Also, the higher the content of fluorine in the polyimide is, the lower the surface tension of a composition containing the polyimide is. Thus, it is difficult to coat such a composition on a substrate and the adhesion of a film comprised of the composition is poor. As a result, film characteristics are deteriorated and the film formed thereby is very fragile. Thus, it is very difficult to put the polylmide into practical use for an optical communications material.